Metal Finishing Guide Book


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Efficiency���It is slightly more efficient than some secondary designs. The advantages of the secondary method are as follows: Reliability���Fewer components mean greater reliability. It has greater voltage safety margin on SCRs. It is less susceptible to line voltage transients. Reversing���It is able to achieve solid-state reversing. PLATING Direct Current Plating Direct current electroplating covers a broad range of processes. These include, but are not limited to, chromium, nickel, copper, zinc, cadmium, silver, and gold. Whereas each of these processes vary somewhat in their particular voltage and current requirements, they all require some form of DC power to deposit the metal out of solution onto the part being plated. A typical DC plating power supply will have a three-phase input of either 230 or 460 V AC. The output will be somewhere in the range of 6 to 18 V and between 50 and 10,000 A. These values will vary depending on whether still- or barrel-plating methods are employed, the type of finish required, and the size of the parts being plated. Direct current plating power supplies are relatively straightforward. The incoming AC is converted to DC by means of the main power transformer and either a primary thyristor/secondary diode or secondary thyristor rectification system. In modern systems, the output voltage and current are controlled by the phase angle of the thyristors. Most rectifiers today are equipped with both automatic voltage control (AVC) and automatic current control (ACC) as standard equipment. In many cases, a variable ramp system is also provided to regulate automatically the rate at which the output is increased from minimum to the desired level. The ripple component of the output at full-rated power is nominally 5% rms of nameplate rating. This will increase as the thyristor���s phase angles are changed to reduce the output. If particular processes demand continuous use of a system phased back, either a properly sized unit should be utilized, or a ripple filter should be installed to bring the ripple component to an acceptable level. Cooling can be by a number of different methods. Forced air and direct water are the most common. Forced air is acceptable when the surrounding environment is relatively clean and free of contaminants. In a forced-air system, air is drawn in through a series of filtered openings in the rectifier enclosure, forced past the internal power-supply components, and exited through an opening, typically in the top of the supply. Air that contains corrosive materials can cause accelerated deterioration inside the power supply, resulting in reduced life and efficiency. If a plating rectifier is situated in an aggressive atmosphere, direct water cooling should be considered. Direct water-cooling systems pass water through a series of cooling passages in the main power transformer and semiconductor heat sinks. Water-cooled systems are more compact than air-cooled designs, and multiple rectifier systems can be placed closer to each other than air-cooled power supplies; however, water-cooled systems are sensitive to contamination and minerals in the supply water, and in these cases, the power supplies may require periodic maintenance to clean the water passages and filters. Pulse Plating Direct current plating deposits metal utilizing a continuous application of ener771

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